Plate Tectonics

Plate tectonics is a scientific theory that describes the large-scale movement and the distribution of the plates that make up the earth’s lithosphere. The plate tectonic theory is built from the concept of the sliding and movement of the continents. The geoscientific community has accepted the plate tectonic theory after obtaining significant proof on the seafloor spreading concept. The theory provides a uniform understanding of mountain-building processes, volcanoes and earthquakes. Also, it has provided an in-depth understanding of the evolution of Earth’s surface and reconstruction of the past of the continents and oceans. Moving further the plate tectonic theory and the tectonic plates definition are explained below.

What are Tectonic Plates?

The outermost layers of the Earth are divided into two - the lithosphere and asthenosphere. This difference is based on the differences in the mechanical properties and the transfer of heat in-between the two layers. These two provide the basis 

Now, thinking about the topic the first two questions that arise are - what is tectonic and what is plate. The tectonic meaning includes all the large-scale activities such as the volcanic activity that takes place on the lithospheric mantle of the earth. And a plate here means the fragments of the lithospheric surface of the earth which are different from each other in various properties. Thus, in order to define tectonic plates or also known as the lithospheric plates, one says that the tectonic plates are the separate and distinct plates made up of lithosphere which ride upon the fluid-like asthenosphere.

The tectonic plates are made up of lithospheric mantle consisting of two types of crustal material: either the oceanic crust which is found below the water of oceans (made up of silicon and magnesium) or the continental crust (made up of silicon and aluminium) which floats on the water or both. An example of a tectonic plate containing both the oceanic crust and the continental crust is the African plate which consists of the continent and the parts of the floor of the Atlantic and the Indian Ocean.

When asked how many tectonic plates are there, one can easily name several of them such as the seven continents - Asia, Africa, Europe, North America, South America, Australia and Antarctica which are the continental plates, and the ones beneath the oceans forming the base of the Pacific, Atlantic and the Indian Oceans are the oceanic plates.

The tectonic plates are known to move and the existence of the current map of the world has come into existence because of the movement of tectonic plates. These lithospheric plates move by a typical range from 10 mm - 40 mm per year (the rate of growth of fingernails) to 160 mm per year (the growth rate of hair). Examples of the slow-moving plate include the Mid-Atlantic ridge while the fast-moving plate includes the Nazca plate. As these earth plates move over the limited space on earth sometimes they separate while sometimes they come into contact with each other. The location where two earth plates meet is called the tectonic plate boundaries. 

The reason for the movement of earth plates and different types of plate boundaries is explained further.

Why Do Plates Move?

Given that there are differences between the physical characteristics of the lithosphere and the asthenosphere, it is widely accepted that the reason for the question, why do plates move, is the relative density of the oceanic lithospheric plate and the relative weakness of the asthenosphere. This difference leads to the creation of specific zones at which the oceanic plate starts submerging into the asthenosphere because of higher density. These are known as subduction zones and it is considered to be one of the driving forces of plate motion. Including this there are other factors that contribute to the movement of the earth plates which are mentioned below:

• One of the driving forces considered for the movement of the lithospheric plates is the mantle dynamics where it is thought that large-scale convection currents in the upper mantle are being passed through the asthenosphere. These convection currents reach the plates which show varying density distribution and affect them differently causing them to move. 

• Another one of the reasons for tectonic plates is the action of gravity. It is thought that there is a slab pull at the subduction zones i.e. the lithospheric plates being pulled into the asthenosphere because of gravity. Hence, in most cases, gravity acts as a secondary driving force for plate movement.

• The third important driving force for plate movement is the rotation of the Earth. Tidal forces and centrifugal forces have been considered to be also the reason for continental drift. These forces, the result of the gravitational pull of the Moon and the Sun, are thought to provide aid or a small push over long periods of time for the movement of the continents.

Although there is proof of the tectonic movement by the seafloor spreading i.e. the creation of new oceanic crusts due to the volcanic eruptions inside the oceans indicating recycling of the oceanic lithospheric plate, there has not been conclusive evidence on the major driving force of the lateral movement of the lithospheric plates. The current movement is still undergoing active research and the movement is said to be a sum of all the above mentioned driving forces, with some forces playing an active role while the other a more passive one. 

Tectonic Plate Boundaries and Its Types

The locations at which the tectonic plates meet or the locations from which they separate are called the tectonic plate boundaries. Depending on the movement of the tectonic plates relative to each other, there are four different types of plate boundaries. These different types of plate boundaries are stated below:

1. Divergent Boundaries: These types of tectonic plate boundaries are formed when two tectonic plates are moving away from one another. This leads to the creation of more rivers or huge water bodies. It is a constructive plate boundary of the four boundaries. 

2. Convergent Boundaries: These ones are formed when two plates collide with each other. In this case, mostly a subduction zone is created and the oceanic plate (heavier in weight) moves below the continental plate (lighter in weight). Sometimes, there is a continental collision when the subduction zone is completely destroyed and its a simple collision. This kind of plate boundary of the four different types of plate boundaries is the most destructive one. At the junction of such a boundary, mountain ranges appear. 

3. Transform Boundaries: In this type of tectonic plate boundaries, the earth plates slide past each other or grind past each other in a lateral movement. It can move to the left or right depending on the point of observation. 

4. Unclear Boundaries: There are plate boundary zones where the interaction between the tectonic plates is unclear and the phenomenon occurring at the belt of such a boundary is not well defined.

Conclusion of the Tectonic Plate Theory

As mentioned at the beginning, if one is to define plate tectonics, it can be said that the Plate tectonics theory or the Tectonic plate theory is a scientific theory describing the movement of the segments of the earth’s lithosphere. It covers a wide range of concepts relating to what are tectonic plates and what are types of tectonic plates, why do they move, and how are they formed. It has been developed by geologists and is an ongoing research field.